Dear Jim
You wrote
> The calendar only specifies how the reference date and time
> are to be interpreted. It says nothing about either the time
> variable values or the decoding that should be used to turn those
> elapsed time values into dates and times. That choice is entirely up
> to the data consumer. If a data producer started with a set of
> Julian calendar dates and created a time variable, and a data user
> prefers to use Proleptic Gregorian dates, there is no problem. One
> is not more correct than the other.
You are right to point to this as a point of disagreement. I thought we had
discussed this earlier e.g. in
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2015/058224.html
I wrote
> Clarify that in the CF convention the choice of "calendar" implies the
> particular set of rules that is used to convert between date-times (YYYY-MM-DD
> hh:mm:ss i.e. sets of six numbers) and time coordinates in units of elapsed
> time since a reference time.
and I believe that this arose from an earlier discussion about this being a
CF-specific use of the term "calendar". Maybe I have misunderstood you now.
I think the data producer is the person who decides what the data means. If
the producer has Julian calendar timestamps and encodes with Julian rules
as a time coordinate variable, the data-user is wrong to decode them with
any other rules into timestamps or interpret them as being in any other
calendar. Why would that be a useful thing to do? I agree with your earlier
posting and email that there is a range of timestamps which refer to the same
points in time in the Gregorian and Julian calendars (long ago, before they
drifted apart) so for that range of dates it would not matter if the data-
user changed the calendar, since they're indistinguishable. But that is a
special case. If you come up to present, a given time-stamp refers to a
different instance in time in the Gregorian and Julian calendars, just like
it does between UTC and GPS calendars. For model calendars, it would be
nonsense for time coordinates encoded in the 360-day calendar to be decoded
in the proleptic Gregorian calendar, for example.
Perhaps we view time coordinates in different ways? I think the timestamps
are the primary information, and the time coordinates are an encoded version.
We do it like that for efficiency of storage, and convenience and robustness
of processing, since string-valued timestamps are relatively awkward. It has
also the great advantage that the encoded time coordinate is also an elapsed
time variable, so it can be used to check monotonicity and for calculations.
This is a common need, since time is a continuous coordinate.
Best wishes
Jonathan
Received on Tue Jun 09 2015 - 11:21:18 BST
